This invention relates to an internal grinder which successively operates at two or more kinds of predetermined infeed speed in a grinding operation, and more particularly to an internal grinder which selects a changing point from one infeed speed to another in response to the grinding wheel diameter change.
Cutting ability of a grinding wheel is influenced very much by various grinding phenomema and parameters, and particularly by cutting angles of abrasive grains relative to the work surface, i.e. by the wheel diameter. This is well demonstrated in an infeed control method which has a predetermined non-constant infeed vs. time characteristic, as shown by a heavy line .alpha. in FIG. 1, and which has a fixed changing point A from rapid infeed (0 to A) to rough grinding infeed (A to B) and a fixed changing point B from the rough grinding infeed to fine grinding infeed (B to C).
When the wheel diameter becomes smaller to some extent, the grinding ability is better so that the actual stock-removal of the workpiece occurs faster as shown by chain line .beta. in FIG. 1 to reach the point t.sub.1 from which the fine grinding starts and the whole grinding process is finished at the point f.sub.1 in a shorter time, with the infeed table returning to c.sub.1. On the contrary, a larger wheel diameter tends to be accompanied by poorer grinding ability so that the actual stock removal occurs slower as shown by the two-dotted chain line .gamma. in FIG. 1 to reach at the starting point t.sub.2 of fine grinding and finish the whole grinding process with at f.sub.2, the infeed table returning to c.sub.2 in a longer time than that required with a small wheel diameter.
A shorter cycle time of the grinding operation as the former case gives a grinding finish with poor cylindricity and rough surface finish, while a longer cycle time as the latter case requires a longer time than necessary.